The 133d-ranked Duan had a chance to serve out the set at 5-3 in her Grand Slam debut. But the 2009 US Open finalist broke back. Then Duan had a set point on Wozniacki’s serve that she couldn’t convert.

Wozniacki broke serve in the next game, then staved off three break points to clinch the win.

Errani feasts

Errani, the 2012 French Open runner-up and a semifinalist at Flushing Meadows last year, needed 51 minutes in Armstrong Stadium to dispatch her 151st-ranked opponent, who lost in qualifying but made it into the main draw when Ayumi Morita withdrew with an injury.

Seventh-seeded Petra Kvitova, the 2011 Wimbledon champ, was pushed to a third set before advancing with a 6-2, 3-6, 6-1 victory over Misaki Doi of Japan in a rare matchup of two lefties. Kvitova had 29 winners but the same number of unforced errors on a windy day.

‘‘I like big matches on the big stadium, but the people — it’s too much crowds for myself, I think,’’ Kvitova said.

Straight shooting

Tomas Berdych, the fifth seed, and No. 10 Milos Raonic also picked up straight-set victories. On a day that American men went 5-2, led by No. 13 John Isner and No. 26 Sam Querrey, a handful of seeded players made quick departures, including No. 15 Nicolas Almagro, No. 25 Grigor Dimitrov, and No. 28 Juan Monaco.

When Isner lost a first-round match in Montreal this month, he dropped to No. 22, marking the first time no American man was listed in the top 20 since the start of the ATP computer rankings in 1973. Isner reached the final the next week in Cincinnati to move back into the top 20.

Raonic won his first Grand Slam match while seeded in the top 10. The Canadian defeated qualifier Thomas Fabbiano of Italy, 6-3, 7-6 (8-6), 6-3.

Inspiration for Nadal

Tennis players around the world have learned plenty from Rafael Nadal. On Tuesday, Nadal learned something from Conner Stroud. The 12-year-old from Spindale, N.C., was born without hips, ankles, femurs, and knees. Encouraged by parents, Stroud has been playing against able-bodied kids in local tournaments — even winning a couple — and inspiring people young and old.

‘‘The most important thing is that he’s happy,’’ Nadal said. ‘‘He’s able to keep practicing the sport. He’s playing tennis. That’s great for him, for the family. That’s a great example that you can be happy even if life doesn’t give you everything. It’s a big example for me and should be a big example for a lot of people.’’